1. Field of Invention
The present invention relates generally to ship wake reduction and, more particularly, to suppressing the visibility of a ship's wake by removing the portion of the boundary layer that produces the easily detected radar image ordinarily associated with the ship's wake.
2. Brief Description of Related Art
The structure of a ship's centerline viscous wake has recently been the subject of considerable interest and study. This interest arises because of the distinctive features visible in the image of the ship's wake obtained from an airborne or earth orbiting synthetic aperture radar (SAR) and from the infrared (IR) signature of the ship's surface wake.
Airborne or earth orbiting synthetic aperture radar (SAR) will, under normal circumstances, detect the surface wake produced by the passage of a ship along the surface of the water. Surface wakes, which trail all surface ships, often extends for many miles behind the ship. The examination of the SAR images of ship wakes reveal a number of features that are common to all surface ship wakes. The usually observed features include a narrow dark band, often extending many miles directly behind the ship, with a bright and gradually expanding border.
David Taylor Research center reports DTRC/SESD-89/05 entitled "A Free-Surface Vorticity Layer Model of the Ships Wake," by Roger J. Furey (August 1989) and DTRC 90/005 entitled "Hydrodynamic Stability and Vorticity in a Ship-Model Wake," by Roger J. Furey (February 1990), incorporated herein by reference, explain how the ship-generated vorticity, noted for its persistence, and the inverse bubble/floating drop structure of the ship's free-surface wake layer are the source of the long-lasting features of the various wake signatures.
A basic rule pertaining to vorticity is that the vorticity moves with the fluid. Consequently, the vorticity generated in the momentum thickness portion of the ship's "thin" boundary layer persists into the surface layer in the ship's track. The references show that, although the ship's viscous wake extends to a depth comparable to the ship's draft, there is a concentration of vorticity, comparable in thickness to the momentum thickness portion of the ship's thin boundary layer, which persists into the ship's wake in the form of a free-surface vorticity layer. The free-surface vorticity layer, which is characterized by its "constant" vorticity, and thereby its rotation, is the most significant characteristic of the viscous wake with respect to free-surface effects.
The free-surface vorticity layer and its interaction with the transverse waves of the ship's Kelvin wake are shown to account for many of the observed features of the wake radar image. The transverse waves of the ship's Kelvin wake provide a low frequency disturbance to the vorticity, stretching the free-surface vorticity layer and generating Rossby waves along the edges. Full-scale ship observations demonstrate that the mechanism of Rossby wave production accounts for Bragg scattering in the SAR imaging in the X-band near the ship's stern through the L-band as the layer moves downstream, thereby accounting for the observed narrow-angle bright lines of the wake's radar image.
The rotational aspects of the free-surface vorticity layer lead to a strong two-dimensional tendency about its rotational axis. Additionally, the energy in the vorticity layer does work on the free-stream which results in an enhanced surface tension and in the absorption of the solutes in the fluid. In the past, it has been believed that the two-dimensional character of the layer, and its structure due to absorption, produce specular reflection of incident SAR waves in the body of the layer away from the direction of the source (and receiver) resulting in no return signal and explaining the dark band in the imagery. However, studies conducted by the inventor, and detailed in the incorporated references, provide a more feasible explanation of the ship wake's dark band radar image.
During monitoring of towing tank model experiments on a scaled ship-model, it was noted that the reflection from the water surface of a directed light beam was quite different within and outside the bounds of the upwelling flow, emanating from the model stern, that makes up the surface of the ship's viscous wake. Within the bounds of the viscous wake, the image of the light did not appear to penetrate the surface layer, but appeared to stop at the surface layer with near total reflection of the light. Outside the bounds of the viscous wake, the light beam penetrated the water to the bottom of the tank. Based on the index of refraction of water, the reflected intensity is expected to be near 2 percent of the incident light. This value correlates with what was observed outside the region of upwelling flow. However, the seemingly total reflection of light within the bounds of the viscous wake was unexpected and is not readily explained by thermodynamic analysis of the quasi-equilibrium state of the surface wake.
Through his analyses, the inventor has determined that the reflecting surface is explained by the generation of drops in the upwelling flow in the model wake. Total reflection of light at an interface between dielectrics can occur when the light beam passes from a medium of given optical density to one of lower density. The passage of light from water to air, such a through a water drop, is such a case. Closed shells of fluid, such as gas shells sometimes known as "inverse bubbles," and floating drops can be produced in a fluid by a number of mechanisms, all of which are present in the upwelling flow at the stern of a ship. A characteristic of these "inverse bubbles" and floating drops is that they appear silvery due to the near total reflection of light. The silvery, bright light patch on the free-surface in the model's wake can be explained by this phenomena.
Three mechanisms have been identified as means of producing floating drops and "inverse bubbles": (1) the break-up of a water jet, (2) rising bubbles subjected to turbulence to form gas shells, and (3) the rupture of rising bubbles on breaking the free-surface. These mechanisms for producing floating drops and "inverse bubbles" can all be found in the upwelling flow at the stern of a ship. Drop production in the upwelling flow at the immediate stern can be related to the break-up of a water jet. The fluid closest to the hull is analogous to that on the periphery of the jet and is thus most likely to break into drops before reaching the peak of the upwelling, i.e., the crest of the first transverse wave behind the ship. This fluid, in transition to the free-surface, has the greatest vorticity. Thus, according to Kelvin's minimum energy theorem, the minimum pressure and maximum velocity will occur here. Additionally, this fluid layer is most susceptible to hull vibrations which may be a significant factor in the break-up of the flow into drops.
Vorticity is greatest in what is initially the laminar sub-layer of the ship's thin boundary layer. The laminar sub-layer subsequently becomes the free-surface vorticity layer at the free-surface/air-water interface of the ship's wake. As stated earlier, a basic rule pertaining to vorticity is that the vorticity moves with the fluid. Therefore, the fluid originating in the laminar sub-layer, upon forming "inverse bubbles" and floating drops at the free-surface interface, retains this maximum vorticity. Consequently, once formed, the "inverse bubbles" and floating drops persist for the duration of the vorticity, thus persisting far downstream in the ship's wake. Thus, the vorticity accounts for the long life of the "inverse bubbles" and floating drops in the viscous wake of a ship.
The existence of "inverse bubbles" in the ship-model wake accounts for the unique manner in which light is reflected from the surface wake behind the ship-model. Furthermore, the retention of vorticity by the "inverse bubbles" provides the mechanism by which the bubbles persist far downstream of the ship's stern. Thermodynamic analysis and observation of the ship's wake indicate increased surface tension and reduced surface temperature in the ship's surface wake. The increased surface tension and reduced surface temperature of the ship's wake are compatible with the physical model of the wake which incorporates these vorticity-retaining "inverse bubbles".
The most notable feature of the SAR image of a ship's wake, the often miles-long dark band directly in the ship's track, is usually attributed to specular reflection of the incident electromagnetic (E-M) waves in the microwave range away from the direction of the source (and receiver) such that no return is associated with that segment of the field of view. The vorticity-retaining "inverse bubbles" model of the surface wake provides a more feasible explanation of the dark band feature.
The inverse bubble/floating drop structure of a ship's surface wake layer, which produces near total reflection of incident visible light, has a different effect when the incident E-M waves are in the microwave range as in the case of incident SAR waves. When the reflective film thickness is much less than the wavelength of the incident E-M waves, a destructive interference can be established such that no return is observed. Although the film thickness of a ship's free-surface vorticity layer may well appear large in relation to the angstroms-long wavelengths of visible light, it appears small in relation to the centimeters-long wavelengths of the microwave region. Thus a destructive interference can be established within the surface layer of the ship's viscous wake. Dielectrics such as water, on the other hand, have a tendency to absorb E-M energy. Salt-water absorbs considerably more E-M energy than fresh-water. However, thermodynamic analysis indicates that the surface layer in a ship's viscous wake is less concentrated in salts than is the water outside the wake. Thus, absorption of E-M energy in this layer is closer to that of fresh water and, consequently, sufficient energy is reflected to produce destructive interference in the ship's wake. Such interference would not result outside the ship's wake. Therefore, the vorticity-retaining "inverse bubble" structure associated with the thin surface wake layer provides a mechanism for producing the dark band in the SAR image regardless of its orientation to the SAR.
The SAR image produced by the ship's wake contributes to easy monitoring of ship traffic at sea. Based on the mission of a particular ship, it may be desirable under certain circumstances to eliminate the easily detectable ship wake. At present, Navy ship's do not possess this capability. The fundamental understanding of the surface flow in the ship's wake provided by the aforementioned reports has lead to the present invention's suppression of tell-tale signs of the wake's SAR imagery.